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Computer Communication Networks (CCN) Network Layer

Computer Communication Networks (CCN) Network Layer
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Dr.ShivJindal,India,Teacher
Published Date:19-07-2017
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Computer Communication Networks (CCN) Network Layer 1Chapter Goals • understand principles behind network layer services: – Internetworking concepts – The network layer – IP – routing (path selection) – how a router works 2 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar Overview • Internetworking • network layer services • IP • routing principle: path selection • hierarchical routing • Internet routing protocols reliable transfer – intra-domain – inter-domain • what‟s inside a router? 3 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar The Internetworking Problem • Two nodes communicating across a “network of networks”… – How to transport packets through this heterogeneous mass ? A B Cloud Cloud Cloud 4 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar The Internetworking Problem • Problems: heterogeneity and scaling • Heterogeneity: – How to interconnect a large number of disparate networks ? (lower layers) – How to support a wide variety of applications ? (upper layers) • Scaling: – How to support a large number of end- nodes and applications in this interconnected network ? 5 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar Heterogeneity: Solutions • Translation (eg: bridges): specify a separate mapping between every pair of protocols (+) No software changes in networks required. () Need to specify N mappings when a new lower layer protocol is added to the list () When many networks, subset = 0 () Mapping may be asymmetric 6 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar Heterogeneity: Solutions – Overlay model: Define a new protocol (IP) and map all networks to IP (+) Require only one mapping (IP - new protocol) when a new protocol is added (+) Global address space can be created for universal addressibility and scaling () Requires changes in lower networks (eg: protocol type field for IP) 7 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar Heterogeneity: Solutions () IP has to be necessarily simple else mapping will be hard. – Even in its current form mapping IP to ATM has proven to be really hard. – Basis for “best-effort” forwarding () Protocol mapping infrastructure needed: address hierarchy, address resolution, fragmentation 8 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar The Internet Network layer Host, router network layer functions: Transport layer: TCP, UDP IP protocol Routing protocols •addressing conventions •path selection •datagram format •RIP, OSPF, BGP Network •packet handling conventions layer routing ICMP protocol table •error reporting •router “signaling” Link layer physical layer 9 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar IP Addressing: introduction • IP address: 32-bit 223.1.1.1 identifier for host, 223.1.2.1 router interface 223.1.1.2 223.1.2.9 223.1.1.4 • interface: connection between host, router 223.1.2.2 223.1.3.27 223.1.1.3 and physical link – router‟s typically have multiple interfaces 223.1.3.2 223.1.3.1 – host may have multiple interfaces – IP addresses associated with 223.1.1.1 = 11011111 00000001 00000001 00000001 interface, not host, 223 1 1 1 router 10 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar IP Addressing - 1 • IP address: 223.1.1.1 – network part (high 223.1.2.1 order bits) 223.1.1.2 223.1.1.4 223.1.2.9 – host part (low order bits) 223.1.2.2 223.1.1.3 223.1.3.27 • What’s a network ? (from IP address LAN perspective) 223.1.3.2 223.1.3.1 – device interfaces with same network part of IP address network consisting of 3 IP networks – can physically reach (for IP addresses starting with 223, each other without first 24 bits are network address) intervening router 11 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar IP Addressing - 2 How to find the 223.1.1.2 networks? 223.1.1.1 223.1.1.4 • Detach each 223.1.1.3 interface from 223.1.7.0 223.1.9.2 router, host • create “islands of 223.1.9.1 223.1.7.1 isolated 223.1.8.1 223.1.8.0 networks 223.1.2.6 223.1.3.27 223.1.2.1 223.1.2.2 223.1.3.1 223.1.3.2 Interconnected system consisting of six networks 12 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar IP Addresses given notion of “network”, let‟s re-examine IP addresses: “class-full” addressing: class 1.0.0.0 to A network 0 host 127.255.255.255 128.0.0.0 to B network 10 host 191.255.255.255 192.0.0.0 to C network host 110 223.255.255.255 224.0.0.0 to multicast address D 1110 239.255.255.255 32 bits 13 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar Some Special IP Addresses • All-0s  This computer • All-1s  All hosts on this net (limited broadcast: don’t forward out of this net) • All-0 host suffix Network Address („0’ means „this’) • All-1 host suffix All hosts on the destination net (directed broadcast). • 127...  Loopback through IP layer 14 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar IP addressing: CIDR - 1 • classful addressing: – inefficient use of address space, address space exhaustion – e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network 15 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar IP addressing: CIDR - 2 • CIDR: Classless InterDomain Routing – network portion of address of arbitrary length – address format: a.b.c.d/x, where x is bits in network portion of address host network part part 11001000 00010111 00010000 00000000 200.23.16.0/23 16 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar Subnet Addressing – External routers need to store entries only for the “network ID” – Internal routers & hosts use subnet mask to identify “subnet ID” and route packets between “subnets” within the “network”. – Eg: Mask: 255.255.255.0 = subnet ID = 8 bits with upto 62 hosts/subnet 17 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar Subnet Addressing (Continued) – Route table lookup: • IF ((Maski & Destination Addr) = = Destinationi) Forward to NextHopi – Subnet mask can end on any bit. – Mask must have contiguous 1s followed by contiguous zeros. Routers do not support other types of masks. 18 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar Route Table Lookup: Example 30.0.0.7 40.0.0.8 128.1.0.9 40.0.0.0 30.0.0.0 128.1.0.0 192.4.0.0 40.0.0.7 128.1.0.8 192.4.10.9 Destination Mask Next Hop 30.0.0.0 255.0.0.0 40.0.0.7 40.0.0.0 255.0.0.0 Deliver direct 128.1.0.0 255.255.0.0 Deliver direct 192.4.10.0 255.255.255.0 128.1.0.9 19 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar IP addresses: how to get one? Hosts (host portion): • hard-coded by system admin in a file • DHCP: Dynamic Host Configuration Protocol: dynamically get address: “plug-and- play” – host broadcasts “DHCP discover” msg – DHCP server responds with “DHCP offer” msg – host requests IP address: “DHCP request” msg – DHCP server sends address: “DHCP ack” msg 20 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar